Analysis and Design of a Multiport Resonant DC-Transformer for Solid-State Transformer Applications

In solid-state transformer (SST) applications, the multiport non-resonant converter became a standard solution for the dc/dc stage, mainly because of its simple power flow control. However, little attention was given to the multiport resonant topologies, which have several benefits in terms of soft-switching and power flow capability when operating as a DC-transformer (DCX). The high performance of the multiport resonant converter has not yet been demonstrated in the literature. Thus, a multiport resonant converter operating as DCX is proposed to enhance the power flow management, efficiency, and cost of the SST, without requiring any additional control strategy. For this purpose, the resonant multiport converter based on CLLC (fully bidirectional) and LLC (partially unidirectional) topologies are analyzed and designed as the basic power electronic building block (PEBB) for the SST's dc/dc stage. Finally, experimental results of a 20 kW multiport DCX are shown to prove the effectiveness of the design approach, and the correctness of the theoretical analysis. Experimental results for the designed DCX of 20 kW have shown a peak efficiency of 97.5 %.

I. Introduction

As defined in the literature, the DCX is a dc/dc converter operating in an open-loop that maintains a fixed voltage ratio independent of the power level. Typically, resonant dc/dc converters operating close to the resonant frequency behave naturally as DCXs. For this particular operation point, the voltage gain is load-independent which provides a tight coupling between its input and output voltages.